Wireless communication system, wireless comunication apparatus and wireless communication method

ABSTRACT

A wireless communication system which performs data transmission using spatially multiplexed streams from a first terminal including N antennas to a second terminal including M antennas (N is an integer of 2 or more and M is an integer of 1 or more) is disclosed. The system includes notifying means, training means, channel matrix estimation means, beamforming information feedback means, transmission weight matrix setting means, and beamforming means.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese PatentApplications JP 2006-124538 and JP 2007-056245 filed in the JapanesePatent Office on Apr. 27, 2006, and Mar. 6, 2007, respectively, theentire contents of which being incorporated herein by reference.

BACKGROUND

1. Technical Field

The invention relates to a wireless communication system, a wirelesscommunication apparatus and a wireless communication method usingspatial multiplexing, and more particularly, to a wireless communicationsystem, a wireless communication apparatus and a wireless communicationmethod, in which a transmitter and a receiver share channel informationto perform closed loop type spatial multiplexing transmission.

In particular, the invention relates to a wireless communication system,a wireless communication apparatus and a wireless communication method,which perform beamforming on the basis of information which is fed backfrom a receiver when a transmitter transmits a packet, and moreparticularly, to a wireless communication system, a wirelesscommunication apparatus and a wireless communication method, whichperform beamforming by feeding back beamforming information between abeamformer and beamformee which are different from each other in thenumber of antennas or the number of supported streams.

2. Background Art

As a system for removing wire in an existing wired communication method,a wireless network is attracting attention. A standard of the wirelessnetwork may be the IEEE (The institute of Electrical and ElectronicsEngineers) 802.11 or the IEEE 802.15.

For example, in the IEEE 802.11a/g, as a standard of a wireless LAN, anorthogonal frequency division multiplexing (OFDM) modulation methodwhich is one of a multi-carrier method is employed. In the OFDMmodulation method, since transmission data is distributed to a pluralityof carriers having orthogonal frequencies and is transmitted, the bandof each carrier becomes narrow, frequency use efficiency is very high,and frequency-selective fading interference is strong.

In addition, in the IEEE 802.11a/g standard, a modulation method foraccomplishing a communication speed of a maximum of 54 Mbps issupported, but a next-generation wireless LAN standard for realizing anew high bit rate is required.

As one of a technology of realizing a high speed of wirelesscommunication, multi-input multi-output (MIMO) communication isattracting attention. This is a communication method in which both atransmitter side and a receiver side respectively include a plurality ofantennas to realize spatially multiplexed streams. The transmitter sideperforms spatial/temporal encoding and multiplexing of plural pieces oftransmission data and distributes and transmits the plural pieces oftransmission data to N transmission antennas through channels. Thereceiver side performs spatial/temporal decoding of reception signalsreceived by M reception antennas through the channels to obtainreception data without crosstalk between the streams (for example, seeJP-A-2002-44051 (Patent Document 1)). Ideally, spatial streamscorresponding to the smaller number (MIN[N, M]) of the transmission andreception antennas are formed.

According to the MIMO communication method, a transmission capacity canincrease according to the number of antennas and a communication speedimprovement can be realized, without increasing a frequency band. Sincethe spatial multiplexing is used, frequency use efficiency is high. TheMIMO method uses channel characteristics and is different from a simpletransmission/reception adaptive array. For example, in the IEEE 802.11nwhich is the extension standard of the IEEE 802.11a/g, an OFDM_MIMOmethod using OFDM in primary modulation is employed. Currently, the IEEE802.11n is being standardized in a task group n(TGn) and a specificationestablished therein is based on a specification established in Enhancedwireless consortium (EWC) formed on October, 2005.

In the MIMO communication, in order to spatially divide a spatiallymultiplexed reception signal y into the stream signals x, a channelmatrix H is acquired by any method and the spatially multiplexedreception signal needs to be spatially divided into a plurality oforiginal streams using the channel matrix H by a predeterminedalgorithm.

The channel matrix H is obtained by allowing a transmitter/receiver sideto transmit/receive existing training sequence, estimating the channelsby a difference between the actually received signal and the existingsequence and arranging propagation channels of a combination oftransmission and reception antennas in a matrix form. When the number oftransmission antennas is N and the number of reception antennas is M,the channel matrix is M×N (row×column) matrix. Accordingly, thetransmitter side transmits N training sequence and the receiver sideacquires the channel matrix H using the received training sequence.

A method of spatially dividing a reception signal is largely classifiedinto an open loop type method in which a receiver independently performsspatial division on the basis of the channel matrix H and a closed looptype method in which a transmitter side gives weights to thetransmission antennas on the basis of the channel matrix to performadequate beamforming toward a receiver to form an ideal spatialorthogonal channel.

As an open loop type MIMO transmission method, there is a zero force(for example, see A. Benjebbour, H. Murata and S. Yoshida, “Performanceof iterative successive detection algorithm for space-timetransmission”, Proc. IEEE VTC Spring, vol. 2, pp. 1287-1291, Rhodes.Greece, May 2001 (Non-Patent Document 2)) or a minimum mean square error(MMSE) (for example, see A. Benjebbour, H. Murata and S. Yoshida,“Performance comparison of ordered successive receivers for space-timetransmission”, Proc. IEEE VTC Fall, vol. 4, pp. 2053-2057, AtlanticCity, USA, September 2001 (Non-Patent Document 3)). The open loop typeMIMO transmission method is a relative simple algorithm for obtainingreception weight matrix W for spatially dividing the reception signalfrom the channel matrix H, in which a feedback operation for sharing thechannel information between the transmitter and the receiver is omittedand the transmitter and the receiver independently perform spatialmultiplexing transmission.

As an ideal one of a closed loop type MIMO transmission method, asingular value decomposition (SVD)-MIMO method using SVD of the channelmatrix H is known (for example, seehttp://radio3.ee.uec.ac.jp/MIMO(IEICE_TS). Pdf (Oct. 24, 2003)(Non-Patent Document 1)). In the SVD-MIMO transmission, a numericalmatrix having channel information corresponding to antenna pairs aselements, that is, a channel information matrix H, is subjected to thesingular value decomposition to obtain UDV^(H). A transmitter side usesV in a transmission antenna weight matrix and transmits a beamformedpacket to a receiver and a receiver side typically gives (UD)⁻¹ as areception antenna weight matrix. Here, D is a diagonal matrix havingsquare roots of singular values λ_(i) corresponding to qualities of thespatial streams in diagonal elements (the subscript i indicates ani^(th) spatial stream). The singular values λ_(i) are arranged in thediagonal elements of the diagonal matrix D in ascending order and powerratio distribution or modulation method allocation is performedaccording to communication quality represented by the level of thesingular value with respect to the streams such that a plurality ofspatial orthogonal multiplexed propagation channels which are logicallyindependent are realized. The receiver side can extract a plurality oforiginal signal sequence without crosstalk and theoretically accomplishmaximum performance.

In the closed loop type MIMO communication system, adequate beamformingis performed when the transmitter transmits the packet, but informationon the channel information needs to be fed back from the receiver sidefor receiving the packet.

For example, in the EWC HT (High Throughput) MAC (Media Access Control)Specification Version V1.24, two kinds of procedures, that is, “implicitfeedback” and “explicit feedback”, are defined as the procedure forfeeding back the information on the channel matrix between thetransmitter and the receiver.

In the “implicit feedback”, the transmitter estimates a backward channelmatrix from the receiver to the transmitter using training sequencetransmitted from the receiver, and a forward channel matrix from thetransmitter to the receiver is computed to perform beamforming on theassumption that bidirectional channel characteristics between thetransmitter and the receiver are reciprocal.

In the “explicit feedback”, the receiver estimates a forward channelmatrix from the transmitter to the receiver using training sequencetransmitted from the transmitter and returns a packet including thechannel matrix as data to the transmitter, and transmitter performs thebeamforming using the received channel matrix. Alternatively, thereceiver computes a transmission weight matrix for allowing thetransmitter to perform the beamforming from the estimation channelmatrix and returns a packet including the transmission weight matrix asthe data to the transmitter. In the explicit feedback, since the weightmatrix is computed on the basis of the estimated forward channel matrix,it may not be assumed that the channels are reciprocal.

In view of packet transmission, the transmitter is an initiator and thereceiver is a receiver. However, in view of beamforming, the initiatorfor transmitting the packet is a beamformer and the receiver forreceiving the beamformed packet is a beamformee. Communication from thebeamformer to the beamformee is referred to as “forward” andcommunication from the beamformee to the beamformer is referred to as“backward”. For example, when an access point (AP) transmits a dataframe to a client terminal (STA) as the beamformer, the access pointperform the beamforming on the basis of the channel informationtransmitted from the client in the explicit feedback.

FIG. 14 shows a state where the beamformee estimates the channel matrixexcited by a training signal transmitted from the beamformer. In thesame drawing, a STA-A having three antennas is the beamformer and aSTA-3 having two antennas is the beamformee and feedback is performed onthe basis of a CSI format. In the below-described description orequations, a subscript AB indicates forward transmission from the STA-Ato the STA-B. A numerical subscript corresponds to the antenna number ofthe corresponding terminal.

The training sequence transmitted from the antennas of the STA-A are(t_(AB1), t_(AB2), t_(AB3)) and the signals received by the antennas ofthe STA-A through a channel H_(AB) are (r_(AB1), r_(AB2)), the followingequation is obtained.

$\begin{matrix}{\begin{pmatrix}r_{{AB}\; 1} \\r_{{AB}\; 2}\end{pmatrix} = {H_{AB}\begin{pmatrix}t_{{AB}\; 1} \\t_{{AB}\; 2} \\t_{{AB}\; 3}\end{pmatrix}}} & (1)\end{matrix}$

where, the channel matrix H_(AB) is a 2×3 matrix and expressed by thefollowing equation. But, h_(ij) is a channel characteristic value ofJ^(th) antenna of the STA-A to i^(th) antenna of the STA-B.

$\begin{matrix}{H_{AB} = \begin{pmatrix}h_{11} & h_{12} & h_{13} \\h_{21} & h_{22} & h_{23}\end{pmatrix}} & (2)\end{matrix}$

When the channel matrix H_(AB) is subjected to singular valuedecomposition, the following equation is obtained. Here, U_(AB) is amatrix having an inherent normalized vector of H_(AB)H_(AB) ^(H), V_(AB)is an inherent normalized vector of H_(AB) ^(H)H_(AB) and D_(AB) is adiagonal matrix having a square root of an inherent vector ofH_(AB)H_(AB) ^(H) or H_(AB) ^(H)H_(AB) as the diagonal elements. Inaddition, U_(AB) and V_(AB) are unitary matrices and complex conjugatetransposed matrices thereof become inverse matrices.

H _(AB) =U _(AB) D _(AB) V _(AB) ^(H)  (3)

The transmission weight matrix necessary for forming the frametransmitted from the STA-A to the STA-B is the matrix V_(AB) obtained byperforming the singular value decomposition with respect to the forwardchannel matrix H_(AB). When the beamformee receives a sounding packet,the beamformee divides the sounding packet into spatial stream trainingsto construct the estimation channel matrix H_(AB). The CSI composed ofMIMO channel coefficients h₁₁, h₁₂, . . . which are elements of thechannel matrix is collected and fed back to the STA-A.

If a transmission vector composed of transmission signals of theantennas of the STA-A is x and a reception signal of the STA-B is y, thereception signal becomes y=H_(AB)x in a case where the beamforming isnot performed (un-steered), but the reception signal y becomes thefollowing equation in a case where the beamforming are performed by thetransmission weight matrix V_(AB) (steered).

$\begin{matrix}\begin{matrix}{y = {{H_{AB}V_{AB}x} = {{\left( {U_{AB}D_{AB}V_{AB}^{H}} \right) \cdot V_{AB}}x}}} \\{= {U_{AB}D_{AB}x}}\end{matrix} & (4)\end{matrix}$

Accordingly, the STA-B can perform spatial division to the originalstream by multiplying a reception vector including the reception signalsof the antennas by D_(AB) ⁻¹U_(AB) ^(H) as a reception weight.

FIG. 15 shows a frame exchange procedure for transmitting beamformingfrom the access point to the client terminal by the explicit feedback.

This procedure is initiated by the access point which sends the soundingpacket including a CSI feedback request.

The sounding packet includes the training sequence excited by thechannel matrix. Accordingly, when the sounding packet is received, theclient terminal divides the spatial stream training to estimate thechannel matrix H and collects the CSI. The CSI data is included in thepacket as a CSI feedback (CFB) and returned to the access point.

The access point computes the transmission weight matrix for beamformingfrom the received CFB and multiplies the transmission signal by it totransmit the beamformed packet to the client terminal. Even in a placewhere the communication was hard to be accomplished in the past,communication is accomplished at a high transmission rate by thebeamforming.

As described above, in the explicit feedback, the beamformer can receivethe explicit feedback of the estimation channel matrix from thebeamformee. The format of the feedback format of the estimation channelmatrix is largely classified into a case where an MIMO channelcoefficient is sent and a case where a transmission weight matrix V forbeamforming computed by the beamformee.

The former format is called channel state information (CSI). Thebeamformer needs to compute the transmission weight matrix V forbeamforming by constructing the channel matrix H from the received CSIand performing the singular value decomposition.

The latter is classified into a case where the transmission weightmatrix V for beamforming is sent in an uncompressed format and a casewhere the transmission weight matrix V for beamforming is sent in acompressed format. According to the explicit feedback, a processingburden for estimating the channel matrix in the beamformer side and aprocessing burden for calculating the transmission weight matrix fromthe channel matrix are reduced.

FIG. 16 shows a scheme of a HT control field of an MAC frame defined inthe EWC specification. The HTC field has 32 bits, but, among them,22^(nd) to 23^(rd) CSI/steering fields can specify a feedback typereceived from the beamformee in the explicit feedback (see FIG. 17).

As described above, the processing burden of the beamformer whichperforms beamforming with respect to a transmission frame is reduced bythe explicit feedback. However, when the beamformer and the beamformeeare different from each other in the number of antennas or the number ofsupported streams, several problems are caused at the time ofbeamforming.

In a spatial multiplexing type communication apparatus, the dimensionnumber which is allowed by the processing capability including theestimation of the channel matrix H, the computation of the transmissionweight matrix for beamforming, and the multiplication of thetransmission vector and the transmission weight matrix V for beamformingis generally designed according to the number of antennas includedtherein. Accordingly, the transmission weight matrix for beamformingcannot be constructed by spatially dividing a training signaltransmitted from the beamformer having the number of antennas which islarger than an allowable dimension, the transmission weight matrix forbeamforming cannot be computed from the channel matrix which is fed backfrom the beamformee, or the transmission weight matrix for beamformingwhich is fed back from the beamformee cannot be multiplied with thetransmission vector.

First, consider a case where the explicit feedback is performed with aCSI format.

In a case where the number N of antennas of the STA-A is smaller than orequal to the number M of antennas of the STA-B, no problem is speciallycaused in the beamformee side. FIG. 18 shows a state where the explicitfeedback is performed with a CSI format when N=2 and M=3. The STA-Bincludes a processing capability of M streams, and can estimate an M×Nchannel matrix excited by a training signal including N streams and feedback the collected CSI information to the STA-A. The STA-A side cansuppress the fed-back M×N channel matrix to a range of N rows andcompute the transmission weight matrix for beamforming by the singularvalue decomposition from the N×N channel matrix.

However, in a case of N>M, problems are caused. This is because, whenthe STA-B can process only M streams, the STA-B obtains only M×Mestimation channel matrix using M packets although the STA-A sidetransmits the sounding packet for exciting N-dimensional spatial channelmatrix. FIG. 19 shows a state where the explicit feedback is performedwith the CSI format when N=3 and M=2.

In the EWC specification, when the explicit feedback is applied, ascheme of informing information on channel estimation maximum dimensionis defined as one of the capabilities of the beamformee side. It isdefined that the HT terminal corresponding to high-speed transmissiondeclares that it itself is the HT terminal by including a HT capabilityfield in a predetermined management frame.

FIG. 20 shows a format of a HT capability element. In a TxBF (transmitbeamforming) capability field, any HT function of the beamforming isspecified. FIG. 21 shows the configuration of the Tx beamformingcapability field. The Tx beamforming capability field has 32 bits, but,among them, 19^(th) to 20^(th) bits are allocated to the CSI number ofbeamformer antennae, 21^(st) to 22^(nd) bits are allocated to theuncompressed steering matrix of beamformer antennae, and 23^(rd) to24^(th) bits are allocated to the compressed steering matrix ofbeamformer antennae. In these fields, the spatial dimension number ofthe sounding packet which can be received from the beamformer when thebeamformee performs the explicit feedback with each format is described.

However, in the EWC specification, since it is not defined whichsounding packet is transmitted by the beamformer, the STA-A may transmitthe sounding packet for exciting more than M channels even when theSTA-B informs of its own maximum dimension number by the above-describedscheme and thus the STA-B is forced to estimate M×N channel matrix.

As a method of solving such problems without deteriorating thebeamforming characteristics, it may be considered that a channelestimation maximum dimension N_(max) corresponding to a rated maximumnumber of antennas is given to the STA-B as the beamformee (for example,if it is based on the IEEE specification, N_(max)=4).

For example, when the number of antennas of the STA-B is M=2 and therated maximum number of antennas is N_(max)=4, the STA-B can computeonly a 2×2 matrix in consideration of the communication with theterminal having the same number of antenna, but needs to compute a 2×4matrix. In this case, since calculation or processing circuit needs tobe doubled, miniaturization or low cost of the apparatus is hard to berealized.

The same is also applied to the explicit feedback for feeding back thetransmission weight matrix V for beamforming, instead of the CSI format.

In a case where the number N of antennas of the STA-A is smaller than orequal to the number M of antennas of the STA-B, no problem is speciallycaused in the beamformee side. FIG. 22 shows a state where thetransmission weight matrix V for beamforming is fed back by the explicitfeedback when N=2 and M=3. The STA-B includes a processing capability ofM streams, and can estimate an M×N channel matrix excited by a trainingsignal including N streams, compute an N×M transmission weight matrix Vfor beamforming by the singular value decomposition from the estimationchannel matrix, and feed backs the transmission weight matrixinformation to the STA-A. The STA-A side can perform beamforming usingthe fed-back transmission weight matrix for beamforming.

However, in a case of N>M, problems are caused. This is because, whenthe STA-B can process only M streams, the STA-B obtains only an M×Mestimation channel matrix using M packets although the STA-A sidetransmits the sounding packet for exciting N-dimensional spatial channelmatrix. FIG. 23 shows a state where the transmission weight matrix V isfed back by the explicit feedback when N=3 and M=2.

In the EWC specification, when the explicit feedback is applied, ascheme of informing information on channel estimation maximum dimensionis defined as one of the capabilities of the beamformee side (describedabove). However, the STA-A may transmit the sounding packet for excitingmore than M channels even when the STA-B informs of its own maximumdimension number by the above-described scheme and thus the STA-B isforced to estimate M×N channel matrix.

As a method of solving such a problem without deteriorating thebeamforming characteristics, it may be considered that a channelestimation maximum dimension N_(max) corresponding to a rated maximumnumber of antennas is given to the STA-B as the beamformee (for example,if it is based on the IEEE specification, N_(max)=4) and a processingcapability which can compute the transmission weight matrix forbeamforming is given to the obtained N_(max)×N estimation channelmatrix.

For example, when the number of antennas of the STA-B is M=2 and therated maximum number of antennas is N_(max)=4, the STA-B can computeonly a 2×2 matrix in consideration of the communication with theterminal having the same number of antenna, but must compute a 2×4matrix. In this case, since calculation or processing circuit needs tobe doubled, miniaturization, low cost and low power consumption of theapparatus are hard to be realized.

SUMMARY OF THE INVENTION

It is desirable to provide an excellent wireless communication system,wireless communication apparatus and wireless communication method,which are capable of performing communication at a high transmissionrate by a beamformed packet by allowing a terminal which operates as abeamformer to suitably set a transmission weight matrix on the basis ofan estimation channel matrix fed back from a terminal which operates asa beamformee.

It is also desirable to provide an excellent wireless communicationsystem, wireless communication apparatus and wireless communicationmethod, which are capable of suitably performing beamforming by theexplicit feedback without deteriorating beamforming characteristics orincreasing a processing capability of channel estimation or a computingcapability of a matrix for beamforming in the beamformee even when abeamformer and a beamformee are different from each other in the numberof antennas or the number of supported streams.

According to an embodiment of the invention, there is provided awireless communication system which performs data transmission usingspatially multiplexed streams from a first terminal including N antennasto a second terminal including M antennas (N is an integer of 2 or moreand M is an integer of 1 or more), the system including: notifying meansfor notifying the first terminal of a maximum dimension N_(max) at thetime of estimating a channel matrix of the second terminal (N_(max) isan integer of N or less); training means for transmitting a soundingpacket including training sequence for exciting a channel correspondingto the maximum dimension N_(max) from the first terminal to the secondterminal; in a case of N>M, channel matrix estimation means for dividingthe training sequence received by the antennas of the second terminalinto N_(max) or less streams and estimating the channel matrix having Mrows and N_(max) or less columns; a beamforming information feedbackunit which prepares beamforming information necessary for calculating atransmission weight matrix for beamforming in the first terminal on thebasis of the channel matrix estimated in the second terminal and feedingback the beamforming information from the second terminal to the firstterminal; transmission weight matrix setting means for setting thetransmission weight matrix for beamforming at the time of transmittingdata from the first terminal to the second terminal on the basis of thebeamforming information fed back from the second terminal to the firstterminal; and beamforming means for performing beamforming intransmission signals of the antennas of the first terminal using thetransmission weight matrix for beamforming when a data packet istransmitted from the first terminal to the second terminal.

The term “system” described herein indicates a logical set ofapparatuses (or function modules for realizing specific functions) andit is not specially considered whether the apparatuses or the functionmodules are included in a single casing (The same is true in the belowdescription).

As a technology for realizing a high speed of wireless communication,there is an MIMO communication method which includes a plurality ofantenna elements in a transmitter side and a receiver side and realizesspatially multiplexed streams. In particular, in a closed loop type MIMOcommunication system, a terminal of a data packet transmission sideperforms beamforming on the basis of feedback of information on anestimation channel matrix from a terminal of a reception side such thata plurality of spatial orthogonal multiplexed propagation channels whichare logically independent are realized and the receiver side can extracta plurality of original signal sequence without crosstalk, therebytheoretically accomplishing maximum performance.

As a procedure of performing feedback of the channel matrix from theterminal of the reception side to the terminal of the transmission side,for example, two kinds of procedures, that is, “implicit feedback” and“explicit feedback”, are defined in the EWC HT MAC specification. Amongthem, in the explicit feedback, the first terminal which operates as abeamformer performs the beamforming of a transmission packet to performcommunication using the transmission weight matrix for beamforming basedon the channel information fed back from the second terminal whichoperates as a beamformee. It is possible to reduce a processing burdennecessary for performing the beamforming in the beamformer.

However, when the beamformer and the beamformee are different from eachother in the number of antennas or the number of supported streams,there are several problems at the time of the beamforming. This isbecause the terminal having a smaller number of antennas needs toperform channel estimation, calculation of the transmission weightmatrix and the multiplication of the transmission weigh matrix with thedimension number larger than or equal to the number which is consideredat the time of designing.

In particular, in a case where the number N of antennas of thebeamformer is larger than the number M of antennas of the beamformee andthe STA-B can correspond to at most M streams, the STA-B cannot obtainthe estimation channel matrix in spite that the sounding packet forexciting the spatial channel matrix of N dimensions is transmitted fromthe STA-A, because the STA-B can correspond to at most M×M estimationchannel matrix.

Accordingly, in the wireless communication system according to theembodiment of the invention, when the beamforming is performed accordingto the explicit feedback, the maximum dimension N_(max) at the time ofestimating the channel matrix of the second terminal is notified to thefirst terminal and the first terminal transmits the sounding packetincluding the training sequence for exciting the channel correspondingto the maximum dimension N_(max). Here, “corresponding to the maximumdimension N_(max)” does not indicate that the excited spatial dimensionis limited to N_(max)”. Generally, it indicates that the second terminalestimates the channel in a format that an estimation process of adimension larger than M×N_(max) is not performed. As an example, thebelow-described staggered sounding packet may be considered. In thiscase, the entire exciting channel dimension is N(>N_(max)), but thetraining sequence are divided into a part for exciting the channel ofN_(max) dimensions and a part for exciting the channel of N-N_(max)dimensions such that the second terminal can perform channel estimationusing only the part N_(max) which can be processed. That is, even whenthe exciting channel space dimension is larger than N_(max), thesounding packet which is considered such that the channel estimation ispossible in the processing capability in consideration of the maximumdimension N_(max) at the time of estimating the channel matrix of thesecond terminal corresponds to “corresponding to the maximum dimensionN_(max)”.

That is, since the first terminal suppresses the number of streams ofthe channel for exciting the sounding packet according to the processingcapability of the second terminal for estimating the channel matrix, thesecond terminal surely receives the sounding packet in a range of itsown maximum dimension number. In this case, it is possible to reduce thesize of the channel matrix estimation circuit of the second terminal asthe beamformee and to realize low cost or low power consumption of theapparatus.

The first terminal is designed to include the processing capabilitycorresponding to the number of its own streams and includes theprocessing capability such as computation for requesting thetransmission weight matrix for beamforming from the channel matrix ofN×N dimensions or less or multiplication of the transmission vector andthe transmission weight matrix for beamforming of N×N dimensions orless. Accordingly, when the CSI information, that is, M×N_(max) channelmatrix, is fed back from the second terminal, the first terminal cancompute the transmission weight matrix for beamforming. Alternatively,even when the M×N_(max) transmission weight matrix for beamformingcomputed from the channel matrix (in the compressed or uncompressedformat) is fed back from the second terminal, the transmission weightmatrix for beamforming can be multiplied with the transmission vector inthe range of its own processing capability and thus no problem iscaused.

On a protocol according to the EWC specification, in a predeterminedmanagement frame, a capability description field for describing apossible maximum spatial dimension when a beamformee of explicitfeedback receives a packet including training sequence is defined.Accordingly, the notifying means can notify the first terminal of themaximum dimension N_(max) at the time of estimating the channel matrixof the second terminal using the management frame for describing thecapability description field. The management frame is, for example, atype of transmission frame of the beacon which is notified in a frameperiod, a measure pilot, an association response and a re-associationresponse which respond to the request of association from the clientterminal, a probe response which responds to the request of BBSinformation from the client terminal, or an association request andre-association request for requesting network association by the clientterminal (or a communication station other than the access point) and aprobe request for requesting BBS information to the access point.Accordingly, even when the second terminal operates as any one of anaccess point and a client terminal, the notifying means can performnotification.

The beamformer may include a signal for requesting the CSI informationin the sounding packet including the training sequence for exciting thechannel. In particular, the CSI information may be requested byspecifying the feedback method received from the beamformee in theexplicit feedback, in the CSI/Steering field provided in the HT controlfield of the MAC frame. Accordingly, the training means may include arequest signal for requesting feedback of the channel information fromthe first terminal to the second terminal in the sounding packet forexciting the channel.

In the EWC specification, a zero length frame (ZLF) (also called a nulldata packet (NDP) and hereinafter referred to as “ZLF”) dedicated to thesounding packet, which includes only a PHY header part including thetraining sequence for exciting the channel and does not include an MACframe, is defined. Since the ZLF does not have the MAC header, the CSIinformation cannot be requested by the HT control field. In such a case,the training means does not include the signal for requesting the CSIinformation in the sounding packet and requests the CSI information inthe HT control field of a general packet transmitted prior thereto.

It is possible to reduce an operation amount necessary for estimatingthe channel matrix by transmitting the sounding packet in a staggeredformat for temporally dividing a training signal part used for a spacedivision process of a data part and a training signal for exciting achannel of a spatial dimension larger than or equal to the number ofstreams of data from the first terminal to the second terminal.

In particular, the first terminal excites the channel of N_(max) spatialdimensions in a training signal part used for the space division processof the data part and allows a training signal for exciting the channelof N-N_(max) remaining spatial dimensions to be not related to the spacedivision of the signal, with respect to the sounding packet.

In this case, when the second terminal receives the sounding packet, thechannel of N_(max) spatial dimensions is excited to estimate M×N_(max)channel matrix in the training signal part used for the space divisionprocess of the data part, but the training signal for exciting thechannel of N-N_(max) remaining spatial dimensions does not need to beprocessed. Although a part attached to the end of the training is notprocessed in order to excite remaining N-N_(max) channels, no problem iscaused in channel estimation or data symbol demodulation.

If the direct mapping for mapping one antenna branch to eachtransmission stream is performed when the first terminal transmits thesounding packet with the stream suppressed to the maximum dimensionN_(max) or less which is allowed by the second terminal, the beamformingeffect deteriorates. Since the first terminal includes N antennas butdoes not use all the antennas and the beamforming must be originallyperformed with respect to the M×N channel matrix but the dimensionnumber is suppressed to M×N_(max), a beam gain is reduced and thus atransmission diversity gain is also reduced. The transmission power ofN_(max) antenna branches used for transmission increases and distortionof the signal increases in a transmission end.

When the dimension number of the sounding packet is suppressed, forexample, the first terminal may perform conversion for mapping N_(max)spatial streams to all N transmission antenna branches by a spatialexpansion and compensate the deterioration of the characteristics by thetransmission diversity. For example, it is possible to perform themapping to the transmission signals to all N transmission antennabranches by multiplying the sounding packet of N_(max) dimensionsspecified from the second terminal by an N×N_(max) mapping matrix.

Correlation between the transmission antenna branches may not besufficiently reduced by the multiplication of the mapping matrix.Accordingly, the first terminal may give different cyclic shift delayamounts to the transmission antenna branches after the multiplication ofthe mapping matrix. The undesired beamforming may be performed whenidentical or similar signals are transmitted through different spatialstreams. However, it is possible to reduce the correlation between thetransmission antenna branches to reduce the undesired directionalcharacteristics. The cyclic shift delay described herein is an operationfor cutting out a part of the time axis waveform of the OFDM symbol andfitting the part to the opposite end (corresponding to phase rotation ona frequency axis) (see FIG. 10), which is different from the simpledelay of the transmission timing between the transmission antennabranches.

When the first terminal performs the conversion for mapping the N_(max)spatial streams to all the N transmission antenna branches, othermapping to the transmission antenna branches may be performed in thesubcarrier unit, as shown in FIG. 11. In this case, since thecorrelation between the transmission antenna branches is high in thesubcarrier unit, the cyclic shift delay may be used together, asdescribed above.

According to the embodiment of the invention, it is possible to anexcellent wireless communication system, wireless communicationapparatus and wireless communication method, which are capable ofperforming communication at a high transmission rate by a beamformedpacket by allowing a terminal which operates a beamformer to suitablyset a transmission weight matrix on the basis of an estimation channelmatrix fed back from a terminal which operates as a beamformee.

According to the embodiment of the invention, it is possible to providean excellent wireless communication system, wireless communicationapparatus and wireless communication method, which are capable ofsuitably performing beamforming by the explicit feedback withoutdeteriorating beamforming characteristics or increasing a processingcapability of channel estimation or a computing capability of a matrixfor beamforming in the beamformee even when a beamformer and abeamformee are different from each other in the number of antennas orthe number of supported streams.

According to the wireless communication system of the embodiment of theinvention, it is possible to reduce complexity of the circuit or powerconsumption of a communication terminal which is the beamformee bysuppressing the dimension number of the sounding packet transmitted fromthe beamformer according to the processing capability of the beamformeewhen the explicit feedback for feeding back CSI information ortransmission weight matrix for beamforming is performed, even if thenumber of antennas of the beamformer is larger than the number ofantennas of the beamformee.

According to the embodiment of the invention, it is possible to performthe beamforming while maintaining the transmission diversity effect tosome extent by mapping transmission streams to all transmission antennabranches when suppressing the dimension number of the sounding packettransmitted from the beamformer.

The other objects, features and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of an operation procedure (in a casewhere CSI information is fed back) of explicit feedback according to anembodiment of the invention.

FIG. 1B is a schematic diagram of an operation procedure (in a casewhere a transmission weight matrix for beamforming is fed back) of theexplicit feedback according to an embodiment of the invention.

FIG. 2 is a view showing the configuration of a transmitter side of awireless communication apparatus which can operate as a STA-A (or STA-B)in a wireless communication system shown in FIGS. 1A and 1B.

FIG. 3 is a view showing the configuration of a receiver side of thewireless communication apparatus which can operate as the STA-A (orSTA-B) in the wireless communication system shown in FIGS. 1A and 1B.

FIG. 4A is a view showing an example of a transmission operation of aZLF packet.

FIG. 4B is a view showing an example of the transmission operation ofthe ZLF packet.

FIG. 5 is a view showing a format example of a staggered soundingpacket.

FIG. 6 is a view showing a format example of the staggered soundingpacket.

FIG. 7 is a view showing a format example of the staggered soundingpacket.

FIG. 8 is a view showing a format example of the staggered soundingpacket.

FIG. 9 is a view showing a format example of the staggered soundingpacket.

FIG. 10 is a view showing a state where a cyclic shift delay is appliedto an OFDM symbol.

FIG. 11 is a view showing an example of performing mapping totransmission antennas in a subcarrier unit when the number of antennasof the STA-A is N=3 and the number of antennas of the STA-B is M=2.

FIG. 12 is a flowchart illustrating a process when an apparatus operatesas a beamformer on the basis of the explicit feedback procedure.

FIG. 13 is a flowchart illustrating a process when the apparatusoperates as the beamformer on the basis of the explicit feedbackprocedure.

FIG. 14 is a view showing a state where a beamformee estimates a channelmatrix excited by a training signal transmitted from a beamformer.

FIG. 15 is a view showing a frame exchange procedure for transmittingbeamforming from an access point to a client terminal with the explicitfeedback.

FIG. 16 is a view showing a scheme of a HT control field of an MAC framedefined in the EWC specification.

FIG. 17 is a view showing a scheme of a CSI/steering field included inthe HT control field.

FIG. 18 is a view showing a state where the explicit feedback isperformed with a CSI format.

FIG. 19 is a view showing a state where the explicit feedback isperformed with the CSI format.

FIG. 20 is a view showing a format of a HT capability element.

FIG. 21 is a view showing the configuration of a Tx beamformingcapability field included in the HT capability element.

FIG. 22 is a view showing a state where a transmission weight matrix Vfor beamforming is fed back by the explicit feedback.

FIG. 23 is a view showing the state where the transmission weight matrixV for beamforming is fed back by the explicit feedback.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, embodiments of the invention will be described in detailwith reference to the accompanying drawings.

A wireless communication system according to the embodiment of theinvention performs closed loop type MIMO communication and moreparticularly, a terminal of a transmitter side performs beamforming inorder of performing feedback for a channel matrix, for example, in orderof the “explicit feedback” defined in the EWC HT MAC specification. Inthe explicit feedback, a beamformer beamforms a transmission packetusing a transmission weight matrix for beamforming obtained on the basisof an estimation channel matrix fed back from a beamformee so as toperform communication.

However, a processing capability for performing channel estimation or aprocessing capability for computing a matrix for beamforming is given toa terminal in consideration of the number of its own antennas.Accordingly, if the number of antennas of the beamformer is large, thebeamformee may not divide the packet into spatial stream trainings toestimate the channel matrix or obtain the matrix for beamforming fromthe estimation channel matrix because the spatial dimension number islarge even when the beamformer transmits a sounding packet to excite thechannel.

Accordingly, in the wireless communication system according to theembodiment of the invention, when the beamforming is performed by theexplicit feedback, a maximum dimension N_(max) of the beamformee at thetime of the channel matrix estimation is notified to the beamformer, andthe sounding packet transmitted from the beamformer is excited such thatthe spatial dimension number of the channel to be estimated by thebeamformee is suppressed to the maximum N_(max) or less. Accordingly,since the beamformee surely receives the sound packet in a range of itsown capability, the size of a channel matrix estimation circuit can bereduced and low cost or low power consumption of an apparatus can berealized.

FIG. 1A is a schematic diagram of an operation procedure of the explicitfeedback according to the embodiment of the invention. Here, the numberof antennas of a STA-A as the beamformer is 3 and the number of antennasof a STA-B as the beamformee or the maximum spatial dimension number atthe time of computing the transmission weight matrix for beamforming is2. The procedure is performed on the basis of the EWC MAC specification.

The STA-B previously notifies the STA-A that the maximum dimensionnumber at the time of estimating the channel matrix is 2. The STA-Aexcites the channel to a format in which the spatial dimension of thechannel to be estimated by the STA-B is 2×2 and transmits the soundingpacket to the STA-B using two streams.

Since the STA-B receives the sounding packet in the range of its owncapability, a 2×2 forward estimation channel matrix can be easilygenerated. CSI information composed of coefficients of the estimationchannel matrix is prepared and fed back to the STA-A using two streams.

Since the STA-A receives the fed-back CSI information using threeantennas, the information is received in a 3×2 spatial dimension, but isproperty processed because the STA-A is designed to include a processingcapability corresponding to the number of its own streams. When the 2×2channel matrix is extracted from the CSI information, computation suchas singular value decomposition is performed such that a 2×2transmission weight matrix V for beamforming can be easily obtained.

The STA-A multiplies a two-dimensional transmission vector by the 2×2transmission weight matrix V for beamforming to perform beamforming andtransmits two transmission streams in order to transmit a data packet.Alternatively, two transmission streams are mapped to three antennas byspace expansion to perform the beamforming in a state where transmissiondiversity effect is maintained to some extent.

Thereafter, a request of the sounding packet and the channel estimationand the computation of the transmission weight matrix for beamformingdue to the reception of the sounding packet are repeatedly performedwhenever the STA-A performs the beamforming.

Since the STA-B surely receives the sounding packet in the range of itsown capability, the size of the channel matrix estimation circuit can bereduced and low cost or low power consumption of the apparatus can berealized.

FIG. 1B is a schematic diagram of an operation procedure of the explicitfeedback when the transmission weight matrix V for beamforming is fedback, instead of the CSI information. Here, the number of antennas ofthe STA-A is 3 and the number of antennas of the STA-B and the maximumspatial dimension number at the time of computing the transmissionweight matrix for beamforming are 2.

The STA-B previously informs the STA-A that the maximum dimension numberat the time of estimating the channel matrix is 2. The STA-A excites thechannel to a format in which the spatial dimension of the channel to beestimated by the STA-B is 2×2 and transmits the sounding packet to theSTA-B using two streams.

Since the STA-B receives the sounding packet in the range of its owncapability, a 2×2 forward estimation channel matrix can be easilygenerated. The 2×2 transmission weight matrix V for beamforming isobtained by performing the computation such as the singular valuedecomposition from the 2×2 channel matrix and fed back to the STA-Ausing two streams.

Since the STA-A receives the fed-back information using three antennas,the information is received in a 3×2 spatial dimension, but is propertyprocessed to extract the transmission weight matrix V for beamformingbecause the STA-A is designed to include a processing capabilitycorresponding to the number of its own streams.

Since the spatial dimension number 2×2 of the transmission weight matrixV for beamforming is in the range of the capability of the STA-A havingthree antennas, the 2×2 transmission weight matrix V for beamforming canbe easily multiplied with the two-dimensional transmission vector, thebeamforming is performed and two transmission streams are transmitted inorder to transmit a data packet. Alternatively, two transmission streamsare mapped to three antennas by space expansion to perform thebeamforming in a state where transmission diversity effect is maintainedto some extent.

Thereafter, the request of the sounding packet, and the channelestimation and the computation of the transmission weight matrix forbeamforming due to the reception of the sounding packet are repeatedlyperformed whenever the STA-A performs the beamforming.

Since the STA-B surely receives the sounding packet in the range of itsown capability, the size of the channel matrix estimation circuit can bereduced and low cost or low power consumption of the apparatus can berealized.

In the operation procedure shown in FIGS. 1A and 1B, the STA-B needs tonotify the STA-A of the maximum dimension number 2 at the time ofestimating the channel matrix. In the EWC specification, when theexplicit feedback is applied, a scheme of informing of information onthe channel estimation maximum dimension as one of the capability of thebeamformee is determined and can be used.

In the EWC specification, it is defined that the HT terminalcorresponding to the high-speed transmission transmits a HT capabilityelement to declare that it is the HT terminal. The HT terminal mayinclude a HT capability field in a predetermined management frame anddeclare any element of a HT function by the HT capability element.

In the TxBF (transmit beamforming) capability field (see FIG. 21)included in the format of the HT capability element, any HT function ofthe beamforming (see FIG. 20) is specified.

The Tx beamforming capability field has 32 bits, but, among them,19^(th) to 20^(th) bits are allocated to the CSI number of beamformerantennae, 21^(st) to 22^(nd) bits are allocated to the uncompressedsteering matrix of beamformer antennae, and 23^(rd) to 24^(th) bits areallocated to the compressed steering matrix of beamformer antennae. Inthese fields, the spatial dimension number of the sounding packet whichcan be received from the beamformer when the beamformee performs theexplicit feedback with each format is described.

The HT capability element may be included in the predeterminedmanagement frame. For example, when the STA-B operates as the accesspoint, the HT capability field may be included in a type of transmissionframe of the beacon which is notified in each frame period, a measurepilot, an association response and a re-association response whichrespond to the request of association from the client terminal, or aprobe response which responds to the request of BBS information from theclient terminal such that the dimension number of the CSI information isnotified to the STA-A which participates in the network operated by theSTA-B. When the STA-B operates as the client terminal (or acommunication station other than the access point), the HT capabilityfield may be included in a type of transmission frame of an associationrequest and re-association request for requesting network association tothe STA-A which operates as the access point and a probe request forrequesting BBS information to the access point. Accordingly, when theSTA-B operates as the access point or the client terminal, the maximumdimension which is allowed in the CSI information may be notified to theSTA-B, by transmitting the HT capability element.

In the beamforming procedure shown in FIGS. 1A and 1B, the STA-A as thebeamformer includes a signal for requesting the CSI information in thesounding packet including training sequence for exciting the channel. Inparticular, in a CSI/Steering field provided in the HT control field(see FIG. 16) of the MAC frame, a feedback method received from thebeamformee can be specified in the explicit feedback (see FIG. 17).

In the EWC specification, a zero length frame (ZLF) dedicated to thesounding packet, which includes only a PHY header part including thetraining sequence for exciting the channel and does not include an MACframe, is defined. Since the ZLF does not have the MAC header, the CSIinformation cannot be requested by the HT control field. In such a case,the signal for requesting the CSI information is not included in thesounding packet and the CSI information is requested in the HT controlfield of a general packet transmitted prior thereto.

FIG. 4A shows an example of a transmission operation of the ZLF packet.As shown, the ZLF packet is transmitted when a short interframe space(SIFS) or a reduced inter frame space (RIFS) elapses after a generaldata packet is transmitted. In the HT control field in the MAC headerincluded in the general data packet, the CSI request for the subsequentZLF packet is performed by specifying the CSI/Steering field.

In an example shown in FIG. 4B, the STA-A requests of the feedback ofthe CSI information in the data frame for requesting an immediateresponse, but declares that the ZLF is continuously transmitted therein.When the STA-B returns an ACK according to the immediate response, theSTA-A transmits the ZLF when the SIFS elapses after the ACK is received.

Up to now, the STA-A which suppresses the spatial dimension number ofthe channel excited according to the processing capability of the STA-Band transmits the sounding packet was described. As a packettransmission method for suppressing the spatial dimension of thechannel, there is a staggered format.

The staggered packet has a training signal part used for a spacedivision process of a data part and a packet structure for temporallydividing a training signal for exciting a channel of the spatialdimension larger than or equal to the number of streams of data and thereceiver side reduces an operation amount necessary for estimating thechannel matrix.

The STA-A excites the channel of N_(max) spatial dimensionscorresponding to the processing capability of the STA-B in a trainingsignal part used for the space division process of the data part andallows a training signal for exciting the channel of N-N_(max) remainingspatial dimensions to be not related to the space division of thesignal, with respect to the sounding packet. In this case, when theSTA-B receives the sounding packet, the channel of N spatial dimensionsis excited to estimate M×N_(max) channel matrix in the training signalpart used for the space division process of the data part, but thetraining signal for exciting the channel of N-N_(max) remaining spatialdimensions does not need to be processed. Although a part attached tothe end of the training is not processed in order to excite remainingN-N_(max) channels, no problem is caused in channel estimation or datasymbol demodulation.

Now, the procedure of the explicit feedback when the sounding packet ofthe staggered format is used will be described. For simplification ofthe description, although a case where each stream is directly mapped toeach antenna branch will be described, the invention is not limited tothe case.

FIG. 5 shows a format example of the staggered sounding packet when thebeamformee having three antennas transmits data of one stream.

A HT-STF (short training field) is a training symbol for improvingautomatic gain control (AGC) in the MIMO system, which includesQPSK-modulated OFDM signals of 52 tones. A HT-LTF (long training field)is a training symbol for performing the channel estimation for eachinput signal which is spatially modulated in the receiver side, whichincludes BPSK-modulated OFDM signals of 56 tones. These are trainingsignals defined in a HT mode of the EWC specification. A value of −400nsec which is described in the HT-LTF simultaneously transmitted from athird antenna is a cyclic shift delay amount which is provided in orderto avoid unintended beamforming when identical or similar signals aretransmitted through different spatial streams, which shifts and connectsa time axis wavelength of an OFDM symbol sent from the third antenna by−400 nanoseconds.

In the example shown in FIG. 5, one stream is transmitted with a formathaving a data stream, but, with a temporal separation therefrom,training signals for exciting the channel of the remaining spatialdimension are transmitted from the other two antennas which are not usedfor the space division process of the data part.

FIG. 6 shows a format example of the staggered sounding packet when dataof one stream is transmitted from the beamformee having four antennas.In the shown example, one stream is transmitted with the format havingthe data stream and, with a temporal separation therefrom, the trainingsignals for exciting the channel of the remaining spatial dimension aretransmitted from the other three antennas which are not used for thespace division process of the data part. In the current EWCspecification, it is defined that four HT-LTFs are used in the trainingof three streams.

FIG. 7 shows a format example of the staggered sounding packet when dataof two streams is transmitted from the beamformee having three antennas.In the shown example, two streams are transmitted with the format havingthe data stream and, with a temporal separation therefrom, the trainingsignals for exciting the channel of the remaining spatial dimension aretransmitted from the other one antenna which is not used for the spacedivision process of the data part.

FIG. 8 shows a format example of the staggered sounding packet when dataof two streams is transmitted from the beamformee having four antennas.In the shown example, two streams are transmitted with the format havingthe data stream and, with a temporal separation therefrom, the trainingsignals for exciting the channel of the remaining spatial dimension aretransmitted from the other two antennas which are not used for the spacedivision process of the data part.

FIG. 9 shows a format example of the staggered sounding packet when dataof three streams is transmitted from the beamformee having fourantennas. In the shown example, three streams are transmitted with theformat having the data stream and, with a temporal separation therefrom,the training signals for exciting the channel of the remaining spatialdimension are transmitted from the other one antenna which is not usedfor the space division process of the data part. In the current EWCspecification, it is defined that four HT-LTFs are used in the trainingof three streams.

As can be seen from FIGS. 5 to 9, in a wireless communication apparatusin which the number of antennas is two and the maximum number ofestimatable streams is two, the reception of the data part (payload) ofthe packet and the estimation of a necessary channel matrix are in theprocessing capability range which is considered upon designing, when thestaggered sounding packet has the structure shown in FIG. 5, 7 or 8.FIG. 6 shows the staggered sounding packet of one stream, which is notsuitably applied to the invention.

In a wireless communication apparatus in which the number of antennas isthree and the maximum number of estimatable streams is three, thereception of the staggered sounding packet shown in FIGS. 5 to 9 and theestimation of a necessary channel matrix are in the processingcapability range which is considered upon designing. In a wirelesscommunication apparatus in which the maximum number of streams is three,the specification in which four HT-LTFs are received and the channelestimation of three streams is performed therefrom is originallyrequested and no problem is caused in the structure of the apparatus.

As can be seen from FIGS. 5 to 9, when the number N of antennas in ainitiator of the sounding packet (that is, the terminal which operatesthe beamformer in the explicit feedback) is larger than the number M ofantennas in a receiver of the sounding packet (that is, the terminalwhich operates as the beamformee in the explicit feedback), thebeamformee can selectively estimate only necessary M streams withoutperforming the channel estimation of N streams (that is, withoutpreparing an M×N channel matrix), by suitably using the staggeredformat.

If the beamformer includes four antennas and the beamformee includes twoantennas, a circuit burden of the beamformer may not be reduced althoughthe staggered sounding packet of the frame format shown in FIG. 6 isused. No problem is caused in the channel estimation from training(HT-LTF) of a first stream, but, in order to estimate the channel withrespect to one stream, four HT-LTFs of the other three streams which arenot used for the space division process of the data part transmittedwith a temporal separation therefrom needs to be computed. Thus, thesize of the circuit of the beamformee which can support at most twostreams increases.

When the beamformer includes three antennas and the beamformee includestwo antennas, the staggered sounding packet of the frame format shown inFIG. 5 or 7 is used.

When the sounding packet shown in FIG. 7 is transmitted from thebeamformer, the beamformee can estimate the channel of two spatialdimensions necessary for the beamforming using the training signal partin a reception stream of the first to second antennas used for the spacedivision of the data part. Since the reception streams of the third tofourth antennas which are not used for the space division of the datapart transmitted with temporal separation do not need to be processed,the problem that the size of the circuit increases in the beamformeewhich can support at most two streams is not caused. No problem iscaused in the channel estimation or the data symbol demodulationalthough the part attached to the end of the training is not processedin order to excite the third to fourth channels.

When the sounding packet shown in FIG. 5 is transmitted from thebeamformer, the beamformee first estimates the channel using thetraining signal part in a reception stream of the first antennas usedfor the space division of the data part. However, two HT-LTF in each ofthe other two streams, which are not used for the space division of thedata part transmitted with the temporal separation, needs to beprocessed, in order to estimate the channel with respect to one stream.The channel estimation of a 2×2 matrix may be performed from two HT-LTFsand the channel estimation of the other one stream may be performed.However, in this case, since the channel estimation result of a firststream needs to be buffered in another place, the size of the circuitslightly increases compared with the case shown in FIG. 7 which thebuffer is not needed.

As another example, when the beamformer includes four antennas and thebeamformee includes three antennas, the staggered sounding packet of theframe format shown in FIG. 6, 8 or 9 is used.

When the sounding packet of the frame format shown in FIGS. 6 and 8 isused, as described above, the channel estimation is performed without aproblem, but there is a problem that the channel estimation result offirst one or two stream needs to be buffered in another place. When thesounding packet of the frame format shown in FIG. 9 is used, thebeamformee can estimate the channel of two spatial dimensions necessaryfor the beamforming using the training signal part in a reception streamof the first to third antennas used for the space division of the datapart. Since the reception stream of the fourth antenna which is not usedfor the space division of the data part transmitted with the temporalseparation does not need to be processed, the problem that the size ofthe circuit increases in the beamformee which can support at most threestreams is not caused. No problem is caused in the channel estimation orthe data symbol demodulation although the part attached to the end ofthe training is not processed in order to excite the fourth channel.

However, if the direct mapping for mapping one antenna branch to eachtransmission stream is performed when the STA-A transmits the soundingpacket with the stream suppressed to the maximum dimension N_(max) orless which is allowed by the STA-B, the beamforming effect deteriorates.Since the STA-A includes N antennas but does not use all the antennasand the beamforming must be originally performed with respect to the M×Nchannel matrix but the dimension number is suppressed to M×N_(max), abeam gain is reduced and thus a transmission diversity gain is alsoreduced. The transmission power of N_(max) antenna branches used fortransmission increases and distortion of the signal increases in atransmission end.

With respect to such a problem, when the dimension number of thesounding packet is suppressed, for example, the STA-A may performconversion for mapping N_(max) spatial streams to all N transmissionantenna branches by a spatial expansion and compensate the deteriorationof the characteristics by the transmission diversity. For example, it ispossible to perform the mapping of to all N transmission antennabranches by multiplying the sounding packet of N_(max) dimensionsspecified from the STA-B by an N×N_(max) mapping matrix.

For example, when the number of antenna of the STA-A is N=3 and thenumber of antennas of the STA-B is M=2, the dimension number issuppressed to the two-dimensional transmission stream (s₁, s₂) in orderto transmit the sounding packet from the STA-A, but the transmissionsignal can be, for example, mapped to three transmission antennas by amapping matrix E described below.

$\begin{matrix}{E = \begin{pmatrix}a & b \\c & d \\e & f\end{pmatrix}} & (5)\end{matrix}$

That is, in the STA-A, a three-dimensional transmission vector isobtained by the two-dimensional transmission stream (s₁, s₂) by the 3×2mapping matrix E. The STA-B receives the two-dimensional stream (r₁, r₂)by propagating the channel composed of 2×3 dimension shown Equation 2.

$\begin{matrix}{\begin{pmatrix}r_{1} \\r_{2}\end{pmatrix} = {\begin{pmatrix}h_{11} & h_{12} & h_{13} \\h_{21} & h_{22} & h_{23}\end{pmatrix}\begin{pmatrix}a & b \\c & d \\e & f\end{pmatrix}\begin{pmatrix}s_{1} \\s_{2}\end{pmatrix}}} & (6)\end{matrix}$

When the stream conversion is performed at the time of transmitting thesounding packet, the same mapping matrix E needs to be multiplied evenwhen the beamforming is performed to transmit the data stream.

When the transmission streams are mapped to the transmission branchesusing the mapping matrix, there is a typical problem that undesireddirectional characteristics occur due to strong correlation between thesignals when the signals are transmitted from the antennas. In order toavoid this problem, it is preferable that an orthogonal matrix is usedsuch that the correlation between the antenna transmission signals isreduced, if possible.

For example, when two streams are mapped to four transmission antennabranches, the following mapping matrix is multiplied.

$\begin{matrix}\begin{pmatrix}{1/2} & {1/2} \\{1/2} & {{- 1}/2} \\{1/2} & {{- 1}/2} \\{1/2} & {1/2}\end{pmatrix} & (7)\end{matrix}$

As a non-orthogonal example, when two streams are mapped to threetransmission antenna branches, the following mapping matrix ismultiplied.

$\begin{matrix}\begin{pmatrix}{1/\sqrt{3}} & {1/\sqrt{3}} \\{1/\sqrt{3}} & {{- 1}/\sqrt{3}} \\{1/\sqrt{3}} & {1/\sqrt{3}}\end{pmatrix} & (8)\end{matrix}$

The correlation between the transmission antenna branches may not besufficiently reduced by only the multiplication of the mapping matrix.In the STA-A, different cyclic shift delay amounts may be given to thetransmission antenna branches after the multiplication of the mappingmatrix. When identical or similar signals are transmitted throughdifferent spatial streams, an unintended beam may be formed, but thecorrelation between the transmission antenna branches can be reduced bygiving the different cyclic shift delay amounts, thereby reducingundesired directional characteristics.

For example, when a valid symbol length of an OFDM symbol is about 3.2microseconds and a guard interval is about 800 microseconds, the cyclicshift delay amounts, that is, 0 nanoseconds, 50 nanoseconds, 100nanoseconds and 150 nanoseconds, are given to four transmission antennabranches such that the correlation between the transmission signals fromthe antennas can be reduced to reduce occurrence of the directionalcharacteristics. In this case, the cyclic shift delay amounts may bedescribed as described below.

$\begin{matrix}\begin{pmatrix}1 & 0 & 0 & 0 \\0 & {\exp \left( {{- {j2\pi\Delta}_{F}}50n\; \sec} \right)} & 0 & 0 \\0 & 0 & {\exp \left( {1{j2\pi}\; k\; \Delta_{F}100n\; \sec} \right)} & 0 \\0 & 0 & 0 & {\exp \left( {{- {j2\pi}}\; k\; \Delta_{F}150n\; \sec} \right)}\end{pmatrix} & (9)\end{matrix}$

In the above equation, Δ_(F) is a subcarrier interval and k is a serialnumber of the subcarrier. It is possible to simply realize mapping fromthe streams to the transmission antenna branches by multiplying theabove-described mapping matrix E by such a matrix from the left(corresponding to the procedure of giving the cyclic shift delay afterthe multiplication of the mapping matrix), and it is possible to reducethe correlation between the transmission signals of the antennas toreduce the undesired directional characteristics. Such a conversionscheme is called “spatial expansion” defined in the EWC specification.

The cyclic shift delay described herein is an operation for cutting outa part of the time axis waveform of the OFDM symbol and fitting the partto the opposite end (corresponding to phase rotation on a frequencyaxis) (see FIG. 10), which is different from the simple delay of thetransmission timing between the transmission antenna branches.

In the STA-A, when the conversion for mapping the N_(max) spatialstreams to all the N transmission antenna branches, other mapping to thetransmission antenna branches may be performed in the subcarrier unit.FIG. 11 is a view showing an example of performing mapping totransmission antennas in a subcarrier unit when the number of antennasof the STA-A is N=3 and the number of antennas of the STA-B is M=2. Inthis case, since the correlation between the transmission antennabranches is high in the subcarrier unit, the cyclic shift delay may beused together, as described above.

FIGS. 2 and 3 show the configurations of the transmitter and thereceiver of a wireless communication apparatus which can operate as theSTA-A (or the STA-B) in the wireless communication system shown in FIG.1, respectively. The number of antennas of the STA-A is N (the number ofantennas of the STA-B is M) and N (or M) is at most four, for example,on the basis of the IEEE specification, but only two antennas are shownin the figures in order to avoid conflict of the figures.

Transmission data supplied to a data generator 100 is scrambled by ascrambler 102. Subsequently, error correction encoding is performed byan encoder 104. For example, in the EWC HT PHY specification, thescrambling and encoding methods are defined according to the definitionof the IEEE 802.11a. The encoded signal is input to a data division unit106 to be divided into the transmission streams.

In a case where the apparatus operates as the beamformer, the datagenerator 100 generates an MAC frame for describing the request of CSIinformation when performing the explicit feedback. In a case where theapparatus operates as the beamformee, a channel matrix estimation unit216 a of the receiver constructs a data frame including the CSIinformation on the basis of the estimated channel matrix, in response tothe reception of the CSI information request. Alternatively, compressedor uncompressed data frame composed of transmission weigh matrixcoefficients for beamforming calculated from the estimation channelmatrix may be constructed.

In each transmission stream, the transmission signal is punctured by apuncture 108 according to a data rate applied to each stream,interleaved by an interleaver 110, mapped to an IQ signal space by amapper 112, thereby becoming a conjugate baseband signal. In the EWC HTPHY specification, an interleaving scheme expands the definition of theIEEE 802.11a such that the same interleaving is not performed among aplurality of streams. As the mapping scheme, BPSK, QPSK, 16QAM or 64QAMis applied according to the IEEE 802.11a.

A selector 111 inserts the training sequence into the transmissionsignal of each interleaved spatial stream at an adequate timing andsupplies it to the mapper 112. The training sequence include the HT-STFfor improving the AGC in the MIMO system and the HT-LTF for performingthe channel estimation for each input signal which is spatiallymodulated in the receiver side. For example, in the HT-LTF, the trainingsequence of each the transmission stream is inserted with the staggeredformat.

When the beamforming is performed with respect to the transmissionsignal, in a spatial multiplexer 114, a beamforming transmission weightmatrix computation unit 114 a calculates the transmission weight matrixV for beamforming from the channel matrix H using a computation methodsuch as the singular value decomposition and a transmission weightmatrix multiplication unit 114 b multiplies the transmission vectorhaving the transmission streams as the element by the transmissionweight matrix V set by the transmission weight matrix setting unit 114a, thereby performing the beamforming. In order to transmit the soundingpacket, the beamforming is not performed with respect to thetransmission signal.

When the CSI information is fed back from the beamformee, thetransmission weight matrix setting unit 114 a calculates thetransmission weight matrix V for beamforming on the basis of the CSIinformation and sets it to the transmission weight matrix multiplicationunit 114 b. When the compressed or uncompressed transmission weightmatrix V for beamforming is fed back from the beamformee, it is set tothe transmission weight matrix multiplication unit 114 b without change.

An inverse fast Fourier transform unit (IFFT) 115 converts thesubcarriers arranged in a frequency region into a time axis signal.

A stream number adjustment unit 116 adjusts the number of transmissionstreams to the maximum dimension N_(max) or less which is received fromthe STA-B as the beamformee. When the direct mapping is performed, fromthe problem that the beamforming effect deteriorates, the transmissionsignals may be mapped to all the N transmission antenna branches bymultiplication of the N×N_(max) mapping matrix and the deterioration ofthe characteristics may be compensated by the transmission diversity.Occurrence of the undesired directional characteristics may be reducedby giving the different cyclic shift delay amounts to the transmissionbranches. The stream number adjustment unit 116 may be realized by firstmultiplying the transmission vector by the mapping matrix and thenmultiplying it by a matrix for cyclic shift delay.

A guard insertion unit 118 adds a guard interval. A digital filter 120performs band limitation, a DA converter (DAC) 122 converts it into ananalog signal, and an RF unit 124 up-converts the analog signal to anadequate frequency band and transmits it to the channel through eachtransmission antenna.

Meanwhile, the data which reaches the receiver through the channel isanalog-processed in an RF unit 228, converted into a digital signal byan AD converter (ADC) 226, and input to a digital filter 224, in eachreception antenna branch.

Subsequently, a synchronization circuit 222 performs processes includingpacket detection, timing detection and frequency offset correction and aguard removing unit 220 removes the guard interval added to the top ofthe data transmission section. The fast Fourier transform unit (FFT) 218transforms a time axis signal to a frequency axis signal.

A space division unit 216 performs a space division process of thespatially multiplexed reception signal. In particular, a channel matrixestimation unit 216 a divides the spatial stream training included inthe PHY header of the sounding packet and constructs an estimationchannel matrix H from the training sequence.

An antenna reception weight matrix computation unit 216 b calculates anantenna reception weight matrix W on the basis of the channel matrix Hobtained by the channel matrix estimation unit 216 a. In a case wherethe beamforming is performed with respect to the reception packet andthe estimation channel matrix is subjected to the singular valuedecomposition (see Equation 3), the estimation channel matrix becomesequal to an UD and the antenna reception weight W is calculatedtherefrom. A method of calculating the antenna reception weight W is notlimited to the singular value decomposition and a calculation methodsuch as zero forcing or MMSE may be used. An antenna reception weightmatrix multiplication unit 216 c multiplies the reception vector havingthe reception streams as the element by the antenna reception weightmatrix W to perform spatial decoding of the spatial multiplexed signal,thereby obtaining independent signal sequence for each stream.

In the explicit feedback, when the apparatus operates as the beamformee,the CSI information is constructed from the estimation channel matrix Hobtained by the channel matrix estimation unit 216 a and fed back fromthe transmitter side to the beamformer as the transmission data. Whenthe transmission weight matrix V for beamforming is requested from thebeamformer instead of the CSI information, the matrix V obtained byperforming the singular value decomposition with respect to theestimation channel matrix H in the antenna reception weight matrixcomputation unit 216 b is fed back.

A channel equalization circuit 214 performs remaining frequency offsetcorrection and channel tracking with respect to the signal sequence ofeach stream. A demapper 212 demaps the reception signal on the IQ signalspace, a deinterleaver 210 performs deinterleaving, and a depuncture 208performs depuncturing at a predetermined data rate.

A data synthesis unit 206 synthesizes a plurality of reception streamsto one stream. This data synthesis process performs an operation whichis opposed to the data division performed in the transmitter side. Adecoder 204 performs error correction decoding, a descrambler 202performs descrambling, and a data acquiring unit 200 acquires thereception data.

When the apparatus operates as the beamformer, the CSI informationacquired by the data acquiring unit 200 or the compressed/uncompressedtransmission weight matrix V for beamforming is sent to the transmissionweight matrix setting unit 114 a of the transmitter side when theexplicit feedback is performed.

When the wireless communication apparatus operates as the terminal ofthe data transmission side, that is, the beamformee, in the closed looptype MIMO communication, the beamformee notifies the beamformer of itsown allowable maximum dimension N_(max) and the spatial dimension numberof the channel excited by the sounding packet transmitted from thebeamformer is suppressed to the maximum dimension N_(max) or less.Accordingly, since the beamformee surely receives the sounding packet inthe range of its own capability, it is possible to reduce the size ofthe circuit of the channel matrix estimation unit 216 a and to realizelow cost or low power consumption of the apparatus.

FIG. 12 is a flowchart illustrating a process when the wirelesscommunication apparatus shown in FIGS. 2 and 3 operates as theinitiator, that is, the beamformer, on the basis of the explicitfeedback procedure. Here, it is assumed that the number of antennas ofthe beamformer is N and the number of antennas of the beamformee is M.

First, notification of the maximum spatial dimension number N_(max) isreceived from the receiver which operates as the beamformee (step S1).Hereinafter, it is assumed that N_(max)=M The notification is performedby receiving the management frame such as the message of networkassociation or the signal including the HT capability field.

Subsequently, the spatial dimension number of the excited channel issuppressed to M×M and the sounding packet is transmitted to the STA-Busing two streams to obtain the feedback of the transmission weightmatrix V for beamforming or the CSI information (step S2).

Since the channel of M spatial dimensions is excited in the trainingsignal part of the sounding packet such that the beamformee receives itusing M antennas, it is possible to estimate an M×M channel matrix.According to the CSI information request, the CSI information isprepared on the basis of the estimation channel matrix and a packet inwhich the information is carried in a data part returns to thebeamformer. Alternatively, when the feedback of the transmission weightmatrix V for beamforming is requested, the estimation channel matrix issubjected to the singular value decomposition and a packet in which thecoefficient data of the transmission weight matrix V for beamforming isincluded in the compressed or uncompressed format returns to thebeamformer.

When the CSI information is received, the beamformer constructs thechannel matrix (step S3) and obtains the transmission weight matrix forbeamforming at the time of forward data transmission (step S4).Alternatively, the transmission weight matrix V for beamforming may bereceived in the step S3 and the step S4 may be skipped.

The beamforming is performed using the transmission weight matrix forbeamforming in the transmission vector having the transmission signalfrom the antennas as the element and the data packet is transmitted tothe receiver (step S5). It is possible to make an ideal spatialorthogonal channel by applying the transmission antenna weight on thebasis of the channel matrix and performing adequate beamforming which isdirected to the receiver.

FIG. 13 a flowchart illustrating a process when the wirelesscommunication apparatus shown in FIGS. 2 and 3 operates as the receiver,that is, the beamformee, on the basis of the explicit feedbackprocedure. Here, it is assumed that the number of antennas of thebeamformer is N and the number of antennas of the beamformee is M.

First, the maximum spatial dimension number N_(max) of the soundingpacket is notified to the initiator which operates as the beamformer(step S11). Hereinafter, it is assumed that N_(max)=M. The notificationis performed by receiving the management frame such as the message ofnetwork association or the beacon including the HT capability field.

Subsequently, when the sounding packet is transmitted from thebeamformer, the channel of M spatial dimensions is excited. Thebeamformee receives it using M antennas (step S12) and estimates the M×Mchannel matrix (step S13).

The CSI information is prepared from the estimation channel matrix andthe packet in which it is included in the data part returns to thebeamformer (step S14).

Alternatively, when the feedback of the transmission weight matrix V forbeamforming is requested as the sounding packet, in the step S13, theestimation channel matrix is subjected to the singular valuedecomposition to obtain the transmission weight matrix V forbeamforming. In the step S14, the packet in which the coefficient dataof the transmission weight matrix V for beamforming is included in thecompressed or uncompressed format instead of the CSI information returnsto the beamformer.

Since the beamformee surely receives the sounding packet in the range ofits own capability, it is possible to reduce the size of the circuit ofthe channel matrix estimation unit 216 a and to realize the low cost orlow power consumption of the apparatus.

Although the invention will be described in detail with reference to thespecific embodiment, it is apparent to those skilled in the art that theembodiment may be modified or substituted without departing from thescope of the invention.

Although the embodiment in which the invention applies to the MIMOcommunication system according to the EWC specification in the IEEE802.11n is described in the present specification, the scope of theinvention is not limited to the embodiment. As the MIMO communicationsystem using the stream which is spatially multiplexed from a firstterminal including N antennas to a second terminal including M antennas,it is possible to suitably apply the invention to various types ofcommunication systems in which the beamformer performs the beamformingusing the channel information fed back from the beamformee.

Although the embodiment which is applied to the IEEE 802.11n which isextension standard of the IEEE 802.11 is described in the presentspecification, the invention is not limited to the embodiment. Theinvention is applicable to a variety of wireless communication systemsusing an MIMO communication method such as a mobile WiMax (WorldwideInteroperability for Microwave) based on the IEEE 802.16e, the IEEE802.20 which is a high-speed wireless communication standard for amobile object, the IEEE 802.15.3c which is a high-speed wireless PAN(Personal Area Network) using 60 GHz (milliwave) band, a wireless HD(High Definition) which transmitting an uncompressed HD image usingwireless transmission of 60 GHz (milliwave) band, and a fourthgeneration (4G) mobile telephone.

The invention is disclosed as an exemplary aspect and the contentsdescribed in the present specification are restrictively analyzed. Thescope of the invention is defined by claims.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1-32. (canceled)
 33. A wireless communication system which performs datatransmission using spatially multiplexed streams from a first terminalincluding N antennas to a second terminal including M antennas, N beingan integer of 2 or more and M being an integer of 1 or more, the systemcomprising: notifying means for notifying the first terminal of amaximum dimension N_(max) at a time of estimating a channel matrix ofthe second terminal, wherein N_(max) is an integer of M or less;training means for transmitting a packet including training sequence forexciting a channel corresponding to the maximum dimension N_(max) fromthe first terminal to the second terminal; channel matrix estimationmeans for dividing the training sequence received by the antennas of thesecond terminal into N_(max) or less streams and estimating the channelmatrix having M rows and N_(max) or less columns, wherein the channelmatrix is estimated based on a determination that a number of antennas Nof the first terminal is greater than a number of antennas M of thesecond terminal; beamforming information feedback means for preparingbeamforming information necessary for calculating a transmission weightmatrix for beamforming in the first terminal on the basis of the channelmatrix estimated in the second terminal and feeding back the beamforminginformation from the second terminal to the first terminal; transmissionweight matrix setting means for setting the transmission weight matrixfor beamforming at a time of transmitting data from the first terminalto the second terminal on the basis of the beamforming information fedback from the second terminal to the first terminal; and beamformingmeans for performing beamforming in transmission signals of the antennasof the first terminal using the transmission weight matrix forbeamforming when a data packet is transmitted from the first terminal tothe second terminal.
 34. The wireless communication system according toclaim 33, wherein forward spatial multiplexing stream transmission fromthe first terminal to the second terminal is performed according to aprotocol based on predetermined standard specification included in apredetermined management frame, and a capability description field fordescribing a possible maximum spatial dimension is defined when abeamformee of explicit feedback receives a packet including trainingsequence, and wherein the notifying means notifies the first terminal ofthe maximum dimension N_(max) at the time of estimating the channelmatrix of the second terminal using the management frame for describingthe capability description field.
 35. The wireless communication systemaccording to claim 33, wherein the training means includes a requestsignal for requesting feedback of the beamforming information from thefirst terminal to the second terminal in the sounding packet forexciting a channel.
 36. The wireless communication system according toclaim 33, wherein the training means requests feedback of channelinformation from the first terminal to the second terminal in a packetincluding a data symbol which is transmitted prior to the soundingpacket when a dedicated sounding packet for exciting a channel whichdoes not include the data symbol is transmitted.
 37. The wirelesscommunication system according to claim 33, wherein the training meanstransmits the sounding packet in a staggered format from the firstterminal to the second terminal.
 38. The wireless communication systemaccording to claim 37, wherein the training means transmits the soundingpacket, in which a channel of N_(max) spatial dimensions is excited inthe training signal part used for the space division process of the datapart and the training signal for exciting a channel of remainingN-N_(max) spatial dimensions is not related to the space division of asignal, from the first terminal to the second terminal.
 39. The wirelesscommunication system according to claim 38, wherein the channel matrixpreparing means excites the channel of N_(max) spatial dimensions in thetraining signal part used for the space division process of the datapart to estimate a channel matrix having M rows and N_(max) or lesscolumns, but the training signal for exciting the channel of theremaining N-N_(max) spatial dimensions is not processed, when the secondterminal receives the sounding packet.
 40. The wireless communicationsystem according to claim 38, wherein the training means excites thechannel of N_(max) spatial dimensions in the training signal part usedfor the space division process of the data part and transmits thesounding packet which does not include the training signal for excitingthe channel of the remaining N-N_(max) spatial dimensions from the firstterminal to the second terminal.
 41. The wireless communication systemaccording to claim 33, wherein the training means performs conversionfor mapping N_(max) or less transmission streams to all N transmissionantenna branches when a dimension number of the sounding packet of thefirst terminal is suppressed to the maximum dimension N_(max) or less.42. The wireless communication system according to claim 41, wherein thetraining means performs the mapping to the N transmission antennabranches by multiplying transmission streams of N_(max) dimensions by amapping matrix having N rows and N_(max) or less columns.
 43. Thewireless communication system according to claim 41, wherein the mappingto the transmission antenna branches which becomes equal to the trainingmeans is performed with respect to the transmission streams formed usingthe beamforming means.
 44. The wireless communication system accordingto claim 43, wherein orthogonal frequency division multiplexing (OFDM)modulation is used as a wireless communication signal, and wherein thetraining means gives different cyclic shift delay amounts to thetransmission antenna branches after the multiplication of the mappingmatrix.
 45. The wireless communication system according to claim 9,wherein OFDM modulation is used as a wireless communication signal, andwherein the training means performs different mappings in a subcarrierunit when the conversion for mapping the transmission streams of theN_(max) dimensions to all the N transmission antenna branches isperformed.
 46. The wireless communication system according to claim 33,wherein the beamforming information feedback means feeds back thechannel matrix having M rows and N_(max) or less columns estimated inthe channel matrix estimation means from the second terminal to thefirst terminal as CSI information, and wherein the transmission weightmatrix setting means calculates the transmission weight matrix forbeamforming, on the basis of the CSI information received by the firstterminal.
 47. The wireless communication system according to claim 33,wherein the beamforming information feedback means feeds back thetransmission weight matrix for beamforming having M rows and N_(max) orless columns and calculated from the channel matrix estimated by thechannel matrix estimation means from the second terminal to the firstterminal in a compressed or uncompressed format, and wherein thetransmission weight matrix setting means uses the transmission weightmatrix for beamforming received by the first terminal.
 48. A wirelesscommunication apparatus which includes N antennas and performs datatransmission using spatially multiplexed streams to a receiver terminalincluding M antennas, N being an integer of 2 or more and M being aninteger larger of 1 or more, the apparatus comprising: notificationreceiving means for receiving notification of a maximum dimensionN_(max) at a time of estimating a channel matrix of the receiverterminal, wherein N_(max) is an integer of M or less; training means fortransmitting a packet including training sequence for exciting a channelcorresponding to the maximum dimension N_(max) to the receiver terminal;beamforming information receiving means for receiving beamforminginformation necessary for calculating a transmission weight matrix forbeamforming composed of a channel matrix having M rows and N_(max) orless columns, wherein the channel matrix is estimated by the receiverterminal using the packet and based on a determination that a number ofantennas N of the wireless communication apparatus is greater than anumber of antennas M of the receiver terminal; transmission weightmatrix setting means for setting the transmission weight matrix forbeamforming on the basis of the received beamforming information; anddata transmitting means for performing beamforming of the antennas usingthe set transmission weight matrix for beamforming and transmitting adata packet to the receiver terminal.
 49. The wireless communicationapparatus according to claim 48, wherein forward spatial multiplexingstream transmission to the receiver terminal is performed according to aprotocol based on a predetermined standard specification included in apredetermined management frame, and a capability description field fordescribing a possible maximum spatial dimension is defined when abeamformee of explicit feedback receives a packet including trainingsequence is defined, and wherein the notification receiving meansreceives the notification of the maximum dimension N_(max) at the timeof estimating the channel matrix by receiving the management frame fordescribing the capability description field from the receiver terminal.50. The wireless communication apparatus according to claim 48, whereinthe training means includes a request signal for requesting feedback ofthe beamforming information to the receiver terminal in the soundingpacket for exciting a channel.
 51. The wireless communication apparatusaccording to claim 48, wherein the training means requests feedback ofchannel information from the first terminal to the second terminal in apacket including a data symbol which is transmitted prior to thesounding packet when a dedicated sounding packet for exciting a channelwhich does not include the data symbol is transmitted.
 52. A wirelesscommunication method which performs data transmission using spatiallymultiplexed streams to a receiver terminal including M antennas in awireless communication apparatus which includes N antennas N being aninteger of 2 or more and M being an integer of 1 or more, the methodcomprising the steps of: receiving notification of a maximum dimensionN_(max) at a time of estimating a channel matrix of the receiverterminal, wherein N_(max) is an integer of M or less; transmitting apacket including training sequence for exciting a channel correspondingto the maximum dimension N_(max) to the receiver terminal; receivingbeamforming information necessary for calculating a transmission weightmatrix for beamforming composed of a channel matrix having M rows andN_(max) or less columns, wherein the channel matrix is estimated by thereceiver terminal using the packet and based on a determination that anumber of antennas N of the wireless communication apparatus is greaterthan a number of antennas M of the receiver terminal; setting thetransmission weight matrix for beamforming on the basis of the receivedbeamforming information; and performing beamforming of the antennasusing the set transmission weight matrix for beamforming andtransmitting a data packet to the receiver terminal.